1. Field of the Invention
The present invention is related to medical technology for blood flow measurement, and in particular to a blood flow measurement device suitable for controlling the operation of a cardiac pacemaker.
2. Background and Prior Art
Measurement of blood flow velocity can be undertaken transcutaneously (through the skin) or intraluminally (directly within the flow). The present invention relates to techniques for intraluminal flow measurements.
Intraluminal flow measurement is needed in all invasive cardiovascular procedures, e.g., catheterization, pacemaker applications and cardiovascular surgery.
Currently such measurements are undertaken using Doppler methods as well as by thermodilution techniques. Measurements using the Doppler effect function by means of transmission of ultrasound energy in the form of a pulse or a continuous wave into the blood stream, and detection of the Doppler frequency shift of the received, reflected waves. Techniques for undertaking measurements of this type are described in Yugoslavian pending patent application P1852/89, in U.S. Pat. Nos. 4,790,323, 4,771,787, 4,706,681, and 4,697,595, as well as in the paper "Properties of Ultrasound Catheters," B. Breyer and B. Ferik-Petric, in the book "Intracavitary Ultrasound" published by Kluwer, Inc., 1991, edited by N. Bom and N. Roelant.
Using these known techniques, with appropriate frequency filtering, data is obtained regarding the flow in the volume within the field of view of the Doppler system, i.e., in the proximity of a catheter. The advantage of such ultrasound techniques is that it is a direct measurement of the flow, but disadvantages are the relatively high power consumption and sophisticated electronics which are necessary in such Doppler systems.
Another method for bulk flow estimation is that of thermodilution, in which thermometers mounted on a catheter measure the rate of cooling of the blood stream after the injection into the stream of a liquid of a different temperature. The advantage of this method is its simplicity, but disadvantages are the relatively poor accuracy and the necessity of undertaking time averaging of the measurement. This method has been known in medical technology for over 20 years, and is the result of the state of electronics at the time of its development.